Zeolites, or molecular sieves, are described as materials formed by TO4 tetrahedra (T=Si, Al, P, Ge, B, Ti, Sn, etc.) interconnected by oxygen atoms, with pores and cavities of uniform size and shape within the molecular range. These zeolite materials have important applications as catalysts, adsorbents or ion exchangers, amongst others (Martinez et al., Coord. Chem. Rev., 2011, 255, 1558).
The formation of nitrogen oxides (NOx) during the combustion of fossil fuels has become a serious problem in present-day society, since these gases are amongst the main air pollutants. Recently, it has been disclosed that one of the most efficient processes for controlling these gas emissions is the selective catalytic reduction (SCR) of NOx using ammonia as the reducing agent (Brandenberger, et al. Catal. Rev. Sci. Eng., 2008, 50, 492).
In this regard, in recent years it has been disclosed that different silicoaluminate forms of small-pore zeolites with copper atoms introduced therein present high catalytic activity and high hydrothermal stability in the SCR of NOx (Bull, et al. U.S. Pat. No. 7,601,662, 2009; Moliner et al. WO2013159825, 2012). Amongst the different small-pore zeolites, zeolite SSZ-13 (CHA-framework zeolite) with copper atoms introduced therein (Cu-SSZ-13) has been widely used in the literature as a catalyst in the SCR of NOx (Bull, et al. U.S. Pat. No. 7,601,662, 2009). Zeolite SSZ-13 is formed by a tri-directional system of small pores (<4 Å) interconnected by large cavities and, moreover, said crystal structure presents small cages, known as double-6 rings (DA6). In this regard, the great hydrothermal stability of the Cu-SSZ-13 catalyst is due to the co-ordination of the copper atoms in the DA6 present in the large cavities of zeolite SSZ-13 (J. Phys. Chem. C., 2010, 114, 1633).
Another zeolite with structural properties related to those of CHA is SSZ-39 (AEI zeolite structure), which is an silicoaluminate with large cavities connected through a tri-directional system of small pores, and which also presents DA6 in its structure (Wagner, et al. J. Am. Chem. Soc., 2000, 122, 263). Recently, it has been disclosed that the silicoaluminate form of the AEI zeolite structure with copper atoms introduced therein is an active, highly stable catalyst from the hydrothermal standpoint in the SCR of NOx with ammonia (Moliner et al. WO2013159825, 2012), and exhibits an even better catalytic behaviour than the Cu-SSZ-13 catalyst (Moliner et al. Chem. Commun. 2012, 48, 8264).
The first synthesis methodology disclosed for the preparation of the silicoaluminate form of the AEI zeolite structure uses various cyclic quaternary ammoniums with alkyl substituents as organic structure-directing agents (OSDAs) (Zones, et al. U.S. Pat. No. 5,958,370, 1999). In said preparations, the use of silicon oxide and aluminum oxide as sources of silicon and aluminum, respectively, has been claimed for the preparation of the silicoaluminate form of the AEI zeolite structure (Zones, et al. U.S. Pat. No. 5,958,370, 1999). Unfortunately, the silicoaluminate form of materials with the AEI structure obtained by means of said synthesis methodology always present very low synthesis yields (less than 52%), due to the fact that the final crystalline solids have an Si/Al ratio that is much lower than the initial Si/Al ratio introduced into the synthesis gel (see Table 1
TABLE 1Synthesis conditions and synthesis yields obtained by means of thesynthesis process disclosed in the patent “Zones, et al.U.S. Pat. No. 5,958,370, 1999”Si/Al ratio inSi/Al ratioSynthesisthe reactionin the finalyield (%mixturesolidweight)Example 2157.348%Example 165025.551%Example 18308.629%
These very different Si/Al ratios suggest that most of the silicon species introduced in the synthesis remain in solution following the crystallisation process, and do not become a part of the zeolite produced. Therefore, these low synthesis yields prevent the potential commercial application of the silicoaluminate SSZ-39 (AEI framework), despite the fact that the cyclic quaternary ammoniums with alkyl substituents used as OSDAs may be appealing, from an economic standpoint, for the preparation of zeolite SSZ-39, since they can be easily obtained from commercially available pyridine precursors.
The synthesis of the silicoaluminate form of zeolite AEI has been performed with high synthesis yields (greater than 80%) using cyclic quaternary ammoniums as OSDAs and fluoride anions in the synthesis medium (Cao et al., US20050197519, 2005). Unfortunately, the presence of fluorine in the synthesis medium and/or the crystalline material synthesised is not recommendable for potential industrial applications. This is due to the high corrosivity and hazards presented by hydrofluoric acid or fluorinated derivatives when they are used as a reactive source, or as a subproduct formed in post-synthetic steps (for example, in the calcination step). Consequently, it is necessary to develop new efficient synthesis methodologies for the silicoaluminate form of crystalline AEI material in media that are free from fluoride anions. Moreover, this synthesis methodology based on the use of fluoride anions in the synthesis medium results in AEI materials with Si/Al ratios in the final solids greater than 100 (Cao et al., US20050197519, 2005), which suggests a limited incorporation of aluminum into the crystal lattice of the AEI structure. This low incorporation of aluminum species seriously limits the introduction and stabilisation of cation species, such as, for example, Cu2+ (it is worth noting that Al3+ species in tetrahedral coordination in the crystal lattice of zeolite generate a negative charge, which would be responsible for compensating and stabilising the cation species). Therefore, this low quantity of aluminum in the lattice would prevent the preparation of efficient Cu-AEI catalysts for application in the SCR of NOx.
Recently, the preparation of the silicoaluminate form of the AEI crystal structure with high synthesis yields (˜80%) using tetraethylphosphonium cations as OSDAs has been disclosed (Maruo, et al. Chem. Lett., 2014, 43, 302-304; Sonoda, et al. J. Mater. Chem. A, 2015, 3, 857). Unfortunately, this process requires the use of phosphine-derived OSDAs, which presents significant disadvantages. On the one hand, the use of organic molecules derived from phosphines poses serious, inevitable environmental and health problems. On the other hand, the complete elimination of the phosphorous species retained inside the zeolite cavities is very complicated, especially in small-pore zeolites, and the elimination process requires calcination steps at very high temperatures and hydrogen atmospheres for the complete decomposition/elimination of said species (Sonoda, et al. J. Mater. Chem. A, 2015, 3, 857).
As previously discussed, small-pore zeolites substituted with a metal inside the structure, especially small-pore zeolites with copper atoms introduced therein, present an excellent catalytic activity for the SCR of NOx with ammonia or hydrocarbons as the reducing agents in the presence of oxygen. The conventional preparation of this type of metal-zeolites is performed by means of post-synthetic metal ion-exchange processes (Bull, et al. U.S. Pat. No. 7,601,662, 2009).
According to the present invention, we have found a new process for synthesising the silicoaluminate form of the AEI zeolite structure in the absence of harmful compounds such as those mentioned above and with suitable Si/Al ratios. Moreover, it has been discovered that, thanks to the use of zeolites with a high silica content as the only source of Si and Al in the synthesis of these materials, the silicoaluminates obtained have a high silica content, in addition to yields greater than 80%.